使△ABC的三个顶点A，B，C的横坐标保持不变，纵坐标都分别加上2得到△A′B′C′．则△ABC与△A′B′C′相比，其变化是

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A．向上平移了2个单位    

B．向下平移了2个单位  

C.   向左平移了2个单位    

D．向右平移了2个单位